Extended wear hearing aids are intended for patients with low to moderate levels of hearing loss. There are intended to be disposed in the bony region of the ear canal, up to approximately 4 mm from the tympanic membrane. They are intended to remain in place for a period of several weeks or even months without the need to remove the device, and will typically only be removed when the battery is exhausted.
Generically, such devices are subject to a variety of constraints, including but not limited to:                in order to have the highest possible fit rate for the largest number of individuals, the outer dimensions of the device must be minimum, thus the size and thickness of the outer hull or housing must likewise be kept to a minimum;        the hearing aid will be worn for a long period in a moist environment (the inner ear). Thus a very low moisture transmission rate of the packaging is necessary in order to protect the electronic components and to avoid leakage current therein. Furthermore, nickel release from the components of the module must be kept below the release limits defined in ISO 1811: 0.2/0.5 μg Ni/cm2/week;        the hearing aid must not undergo degradation or a change of structural integrity in prolonged contact with sweat and/or cerumen;        skin biocompatibility with regard to ISO 10993-1 must be assured;        the dimensions should not exceed 11.3 mm in length and 3.4×6.4 mm in cross-section.        